1. Field of the Invention
The present invention relates to an optical add/drop multiplexer, and more particularly to an optical add/drop multiplexer including a 2×N arrayed-waveguide grating capable of functioning as a multiplexer and demultiplexer.
2. Description of the Related Art
A passive optical network (PON) includes a single communication line between a central office and a plurality of optical network units. A single optical fiber is coupled between the central office and a remote node installed at a region closest to the plurality of optical network units, and independent optical fibers are coupled between the remote node and subscribers such that the PON is based on a double star structure. The above-described PON is a communication system capable of transmitting a plurality of communication signals through the single communication line. The PON can be based, for example, on a time division multiple access (TDMA), wavelength division multiple access (WDMA), sub-carrier multiple access (SCMA), or other signal division method.
The above-described WDMA-based PON transmits optical signals of different wavelengths assigned to respective subscribers. The WDMA-based PONs are widely used since they can be sufficiently configured using only conventional devices and afford the potential for large communication capacity, communication network security, etc.
The above-described WDMA-based PONs include a uni-directional ring optical communication network based on two optical fibers having a self-healing function, a bi-directional ring optical communication network based on four optical fibers having a self-healing function, and an all-optical network based on a mesh form. The above-described ring optical communication network consists of a plurality of nodes including optical add/drop multiplexers for transmitting (or adding) and receiving (or dropping) optical signals. The optical add/drop multiplexers are important devices needed for implementing the optical communication network, and add or drop an optical signal having a preset wavelength used in optical communication on the optical communication network.
FIG. 1 is a view illustrating a configuration of a conventional optical add/drop multiplexer using a wavelength division multiplexer and a wavelength division demultiplexer. The conventional optical add/drop multiplexer shown includes a demultiplexer 110 for demultiplexing an optical signal, produced by a multiplexing operation, into a plurality of channels having different wavelengths; a plurality of optical switches 130 having one-to-one correspondence with the channels produced by the demultiplexing operation; a multiplexer 120 for multiplexing a plurality of added channels received from the optical switches 130; and a controller (not shown) for outputting, to the optical switches 130, control signals needed for adjusting adding/dropping operations for preset wavelength-based channels.
Conventionally, the demultiplexer 110 is based on a 1×N arrayed-waveguide grating, which includes an input demultiplexing port 111 coupled to an input fiber 101, a plurality of output demultiplexing ports 112 having one-to-one correspondence with a plurality of channels of different wavelengths produced by the demultiplexing operation, etc. An optical signal inputted into the input demultiplexing port 111 is demultiplexed into a plurality of channels having different wavelengths (λi1˜λiN) and then the channels produced by the demultiplexing operation are outputted to the output demultiplexing ports 112. The output demultiplexing ports 112 have one-to-one correspondence with the channels produced by the demultiplexing operation.
Analogously, the multiplexer 120 is typically based on an N×1 arrayed-waveguide grating including a plurality of input multiplexing ports 121 forming input paths for added or bypass channels, an output multiplexing port 122 coupled to an output fiber 102 for externally outputting an optical signal produced b y the multiplexing operation, etc. The multiplexer 120 multiplexes the added or bypass channels inputted into the input multiplexing ports 121.
Each of the optical switches 130 is based on a 2×2 optical switch having the first and second input ports 131 and 132 and the first and second output ports 133 and 134. The optical switches 130 have one-to-one correspondence with the output demultiplexing ports 112 of the demultiplexer 110 and the input multiplexing ports 121 of the multiplexer 120.
The first input ports 131 are coupled to the output demultiplexing ports 112, such that the channels having wavelengths (λi1˜λiN) produced by the demultiplexing operation are inputted into the optical switches 130. The second input ports 132 input the added channels into the optical switches 130. The added channels inputted into the second input ports 132 have wavelengths (λj1˜λjN) different from the wavelengths (λi1˜λiN) produced by the demultiplexing operation of the demultiplexer 110.
The first output ports 133 have one-to-one correspondence with the input multiplexing ports 121 of the multiplexer 120, such that the added or bypass channels are outputted. The second output ports 134 externally output the channels to be dropped.
In a bypass operation 130a of the optical switch 130, a bypass channel among the channels having wavelengths (λi1˜λiN) inputted into the first input port is outputted to the input multiplexing port 121 of the multiplexer 120 through the first output port. In an adding or dropping operation 130b of the optical switch 130, an added channel among the channels having wavelengths (λj1˜λjN) is outputted to the first output port 133 according to an adding operation. Channels to be dropped, having the wavelengths (λi1˜λiN) and inputted into the first input ports 131, are externally outputted through the second output ports 134 according to a dropping operation.
Problematically, the two arrayed-waveguide gratings used as a multiplexer and demultiplexer of the add-drop multiplexer must be controlled to assume the same wavelength transmission characteristics.
In avoiding this problem use can be made of a loop-back type optical add/drop multiplexer based on a N×N arrayed-waveguide grating or a fold-back type optical add/drop multiplexer based on a 2×N arrayed-waveguide grating capable of performing functions of the multiplexer and demultiplexer. However, the loop-back type optical add/drop multiplexer includes a plurality of arrayed-waveguide grating ports which increases cross talk between input/output channels, and the fold-back type optical add/drop multiplexer is limited in the number of channels capable of being processed.